The present invention relates to a diagnostic apparatus for detecting a degradation failure of an exhaust gas sensor disposed in an exhaust passage of an internal-combustion engine (hereinafter referred to as an “engine”).
An exhaust gas sensor is generally disposed in an exhaust passage of an engine of a vehicle in order to measure constituent elements of an exhaust gas. The exhaust gas sensor produces outputs representing air-fuel ratio of the exhaust gas. Based on the output value, an electronic control unit of the engine controls the air-fuel ratio of the fuel to be supplied to the engine. Therefore, when the exhaust gas sensor does not produce outputs reflecting a correct air-fuel ratio due to its degradation failure, the control unit cannot perform a correct control of the air-fuel ratio upon the engine.
There are disclosed some techniques for detecting a degradation failure of such exhaust gas sensor. The Japanese Patent Application Unexamined Publication (Kokai) No. HEI7-145751 and the U.S. Pat. No. 5,325,711 disclose a technique for generating a fuel signal by modulating a rectangular waveform, and processing the output of an oxygen sensor representing exhaust gas for determining an operating condition of the oxygen sensor.
However, in the above-referenced technique, a fuel amount indicated by a modulated rectangular waveform is injected into the engine and a response from the engine is used. A response, which is output responsive to the modulated rectangular waveform containing various frequency components, tends to be influenced by noises. Because such response signals are influenced by operating conditions of the engine, air-fuel ratio variation that may be produced during a transient operation, the frequency of the output signal for evaluating the sensor condition can hardly be kept at a constant level. Therefore, when the sensor condition is evaluated based on such output, evaluation precision may deteriorate. On the other hand, precision of the air-fuel ratio control is getting more important than before as emission control is enhanced and the amount of precious metals carried by the catalyst need to be reduced. Accordingly, in order to suppress an increase of the exhaust gas constituent elements due to the characteristic degradation failure of the exhaust gas sensor, it is required to improve the detection precision more than before and it is also required to suppress the increase of the exhaust gas constituent elements during the degradation detection process.
Thus, it is an objective of the present invention to provide a failure diagnostic apparatus for an exhaust gas sensor, which enables a further improvement of detection precision upon a deterioration failure of the exhaust sensor as well as a minimization of an increase of exhaust gas constituent elements during a degradation detection process.